High-strength Cr—Mo steels and weld beads (weld metals) thereof for use in steam boilers and chemical reactors are exposed to a high-temperature and high-pressure environment during use. They require not only basic properties such as strength and toughness, but also heat resistance (high-temperature strength), stress-relief cracking resistance [resistance to intergranular cracking during a stress-relief heat treatment (SR heat treatment)], and temper embrittlement resistance (resistance to embrittlement during use in a high-temperature environment) at high levels. Recent apparatuses have larger sizes and larger wall thicknesses. Welding on these large-sized apparatuses has been performed with an increasing heat input for better operation efficiency. Such increasing welding heat input will generally cause weld beads to have a coarsened microstructure and inferior toughness (inferior temper embrittlement resistance). To prevent this, weld metals of high-strength Cr—Mo steels require toughness and temper embrittlement resistance at further higher levels.
Various techniques have been proposed while focusing attention on toughness and temper embrittlement resistance of weld metals formed upon welding of high-strength Cr—Mo steels.
Typically, Patent literature (PTL) 1 discloses a technique relating to a weld metal having various properties at certain levels. The weld metal is obtained by minutely specifying chemical compositions of a base steel sheet and a welding material (welding consumable), and welding conditions. Some working examples according to this technique, however, have unsatisfactory toughness after a temper embrittling treatment (step cooling) in terms of vTr′5.5 of at best −41° C., although having satisfactory toughness after a stress relief heat treatment (SR heat treatment) in terms of vTr5.5 of −50° C. The term “vTr′5.5” refers to a temperature at which a sample after the step cooling has an absorbed energy of 5.5 kgf·m. The term “vTr5.5” refers to a temperature at which a sample after the SR heat treatment has an absorbed energy of 5.5 kgf·m.
PTL 2 proposes a technique relating to a coated electrode including a core wire and a coating flux. The technique relationally specifies contents of C, Mn, and Ni while maintaining yields of the core wire and the coating at certain levels so as to improve toughness, strength, and heat resistance. The technique, however, fails to give consideration to temper embrittlement resistance.
Independently, to provide weld metals that excel in toughness, strength, temper embrittlement resistance, and stress-relief cracking resistance, PTL 3 and PTL 4, for example, propose techniques of specifying chemical compositions of solid wires and bonded fluxes, and welding conditions (heat input). Some working examples according to these techniques have satisfactory toughness both after an SR heat treatment and after a temper embrittling treatment (step cooling). Specifically, they have a vTr55 and a vTr′55 of each lower than −50° C. The vTr55 indicates toughness of a sample after an SR heat treatment and refers to a temperature at which the sample after the SR treatment has an absorbed energy of 55 J. The vTr′55 indicates toughness of a sample after a temper embrittling treatment (step cooling) and refers to a temperature at which the sample after the step cooling has an absorbed energy of 55 J. The working examples, however, each have a difference ΔvTr55 (=vTr′55−vTr55) of 8° C. or greater. This indicates that the techniques fail to sufficiently suppress temper embrittlement.
PTL 5 proposes a technique of controlling a chemical composition, particularly amounts of impurity elements, of a weld metal to help the weld metal to have better toughness, strength, and stress-relief cracking resistance. The technique, however, fails to give consideration to temper embrittlement resistance.
PTL 6 proposes a technique of controlling chemical compositions of a core wire and a coating flux of a welding electrode for use in shielded metal arc welding so as to give a weld metal having better toughness and higher strength. The technique, however, fails to give consideration to temper embrittlement resistance. In addition, the technique is significantly limited in operation because a designed welding heat input is small.
PTL 7 and PTL 8, for example, propose techniques of controlling chemical compositions of a core wire and a coating flux of a welding electrode for use in shielded metal arc welding so as to give weld metals having better toughness and higher strength. Weld metals according to the techniques have toughness and temper embrittlement resistance both at high levels. In view of recommended welding conditions, however, the techniques fail to sufficiently support increase in welding heat inputs. This is because the technique disclosed in PTL 7 specifies a weld metal in shielded metal arc welding and recommends a welding current of from about 140 to about 190 A (at a core wire diameter φ of 4.0 mm); and the technique disclosed in PTL 8 specifies a weld metal in submerged arc welding and recommends a heat input of from about 2.0 to about 3.6 kJ/mm.